The disclosure relates gas turbine engines. More particularly, the disclosure relates to thermal barrier coatings for gas turbine engines.
Gas turbine engine gaspath components are exposed to extreme heat and thermal gradients during various phases of engine operation. Thermal-mechanical stresses and resulting fatigue contribute to component failure. Significant efforts are made to cool such components and provide thermal barrier coatings to improve durability.
Exemplary thermal barrier coating systems include two-layer thermal barrier coating systems. An exemplary system includes NiCoCrAlY bondcoat (e.g., air plasma sprayed (APS), low pressure plasma sprayed (LPPS), or cathodic arc deposited) and yttria-stabilized zirconia (YSZ) (or gadolinia-stabilized zirconia (GSZ)) thermal barrier coating (TBC) (e.g., air plasma sprayed (APS) or electron beam physical vapor deposited (EBPVD)). Prior to and while the barrier coat layer is being deposited, a thermally grown oxide (TGO) layer (e.g., alumina) forms atop the bondcoat layer. As time-at-temperature and the number of cycles increase, this TGO interface layer grows in thickness. An exemplary YSZ is 7 weight percent yttria-stabilized zirconia (7YSZ).
Exemplary TBCs are applied to thicknesses of 1-40 mils (0.025-1.0 mm) and can contribute to a temperature reduction of up to 300° F. (167° C.) at the base metal. This temperature reduction translates into improved part durability, higher turbine operating temperatures, and improved turbine efficiency.
Separately, the material known as didymium is used as safety glasses in the glassblowing and blacksmithing industries due to advantageous selective light-filtering properties. U.S. Pat. No. 3,758,417 discloses use as part of a catalyst system. Commercially, didymium ore is obtained from the mineral monazite. The mineral bastnaesite could also be used as source from which didymium ore mixtures are derived.
Monazite is rare earth phosphate combination of approximately 70 percent rare earth oxide (REO). The exact process by which monazite is refined into rare earth ore varies slightly by the deposit from which it is refined. In general, monazite is found as a minor constituent within placer deposits of heavy minerals. Monazite is commonly found intermixed with sillimanite, garnet, and magnetite. Major minerals commonly found with monazite include ilmenite, rutile, zircon, and quartz. Separation of the heavy mineral deposits is accomplished through a series of separation sequences that exploit the small differences in mass, magnetic susceptibility, and electrochemical properties. The number and order of operations is predicated on the source of the deposit and purity of the heavy mineral deposits.
Recovery of REO from monazite involves a complex series of operations by which the rare earths are separated from the radioactive components of the mineral. Monazite is dissolved into solution using caustic soda (NaOH). This mixture is then washed and filtered. Two byproducts are evolved from this step are a mixed rare earth (RE)-thorium-uranium hydroxide and a filtrate containing sodium phosphate. Hydrochloric acid is then added to the RE-Th-U hydroxide solution. The solution is subsequently filtered and washed to separate the radioactive constituents, uranium and thorium, from the desired RE components. The filtrate from this process is neutralized by chemical processing to yield a RE chloride mixture. The remaining liquid fraction is treated either with caustic soda and/or sodium bicarbonate to form additional RE hydroxide or RE carbonate. Similar processing is also used to digest bastnasite RE minerals, however additional processing steps are included to remove additional elemental contaminants from the desired RE elements.
Cerium is the first RE to be removed from the mixture of lanthanide elements. This can be accomplished by drying the rare earth hydroxide mixture and then oxidizing cerium (III) to cerium (IV) in the presence of ozone. The mixture is then dissolved in nitric acid. The subsequent mixture is filtered leaving a cerium-free RE solution and cerium (IV) dioxide filtrate. The remaining mixture of rare earths can be further processed by a series complex ion exchange and digestion methods to separate the mixture into each elemental constituent or the mixture can be thermally treated in an oxidizing environment to convert the mixture to the didymium oxide ore mixture noted in Table III below. See, e.g., Extractive Metallurgy of Rare Earths, C. K. Gupta and N. Krishnamurthy, CRC Press 2004.
U.S. Pat. No. 6,863,999 identifies use of a lanthanum monazite phosphate in a thermal barrier coating.